In vapour-compression refrigeration or heat pump systems the refrigerant circulates in the system and undergoes phase change and pressure change. In the system a refrigerant gas is compressed in the compressor to achieve a high pressure refrigerant gas, the refrigerant gas is fed to the condenser (heat exchanger), where the refrigerant gas is cooled and condensates, so the refrigerant is in liquid state at the exit from the condenser, expanding the refrigerant in the expansion device to a low pressure and evaporating the refrigerant in the evaporator (heat exchanger) to achieve a low pressure refrigerant gas, which can be fed to the compressor to continue the process.
However, in some cases refrigerant in the gas phase is present in the liquid refrigerant conduits caused by boiling liquid refrigerant. This refrigerant gas in the liquid refrigerant conduits is denoted “flash gas”. When flash gas is present at the entry to the expansion device, this seriously reduces the flow capacity of the expansion device by in effect clogging the expansion device, which impairs the efficiency of the system. The effect of this is that the system is using more energy than necessary and possibly not providing the heating or cooling expected, which for instance in a refrigerated display cabinet for shops may lead to warming of food in the cabinet, so the food must be thrown away. Further the components of the system will be outside normal operating envelope. Because of the high load and low mass flow of refrigerant when flash gas is present, the compressor may be subject to overheating, especially in the event that misty oil in the refrigerant is expected to function as lubricant the compressor will undergo a lubrication shortage causing a compressor seizure.
Flash gas may be caused by a number of factors: 1) the condenser is not able to condense all the refrigerant because of high temperature of the heat exchange fluid, 2) there is a low level of refrigerant because of inadequate charging or leaks, 3) the system is not designed properly, e.g. if there is a relatively long conduit without insulation from the condenser to the expansion device leading to a reheating and possibly evaporation of refrigerant, or if there is a relatively large pressure drop in the conduit leading to a possible evaporation of refrigerant.
A leak in the system is a serious problem, as the chosen refrigerant may be hazardous to the health of humans or animals or the environment. Particularly some refrigerants are under suspicion to contribute in the ozone depletion process. In any event the refrigerant is quite expensive and often heavily taxed, so for a typical refrigerated display cabinet for a shop recharging the system will be a considerable expense. Recently a shop having refrigerated display cabinets lost half of the refrigerant in the refrigeration system before it was detected that the refrigeration system had a leak, and recharging of the system was an expense of 75,000 dkr, approximately 10,000 $.
A known way to detect flash gas is to provide a sight glass in a liquid conduit of the system to be able to observe bubbles in the liquid. This is labour and time consuming and further an observation of bubbles may be misleading, as a small amount of bubbles may occasionally be present even in a well functioning system.
Another way is to indirectly detect flash gas by triggering an alarm when the expansion device is fully open, e.g. in the event that the expansion device is an electronic expansion valve or the like. In this case a considerable number of false alarms may be experienced, as a fully open expansion device may occur in a properly functioning system without flash gas.